Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT

Subject: Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT
From: Joel de Guzman (djowel_at_[hidden])
Date: 2016-03-02 21:12:05

On 03/03/2016 9:43 AM, Jens Weller wrote:
> Hello,
>
> Code below, first what I'm trying to do is...
> assign a value to a adapted struct (BOOST_FUSION_ADAPT_ADT), which works fine if both are of the same type or convertible:
>
> template<int I, class Seq, class V>
> typename std::enable_if< std::is_convertible< typename fusion::result_of::at_c<Seq, I>::type, V>::value ,void>::type assign(Seq& s,  V& v)
> {
>      fusion::get<I>(s) = v;
> }
>
> Some of my input is string, and the struct member is bool, unsigned int etc. In this case I'd need to know the type to convert from string.
>
> boost::fusion::get returns the attribute_proxy for the adapted struct, so does at_c<>::type.
>
> template<int I, class Seq, class V>
> typename std::enable_if< !std::is_convertible< typename fusion::result_of::at_c<Seq, I>::type, V>::value /*&& !std::is_same<typename std::decay<V>::type,std::string>::value*/ ,void>::type assign(Seq& s, V& v)
> {
>      std::stringstream ss;
>      ss << v;
>      ss >> fusion::get<I>(s);
> }
>
> Obviously this does not compile. Is there any trick to get to the type of the proxy?
>
> One idea I have is decltype(fusion::get<I>(s).get()), but not sure if I can get this to work in an enable_if...

Hi Jens,

Try attribute_proxy::type. It's documented here:
http://www.boost.org/doc/libs/1_50_0/libs/fusion/doc/html/fusion/notes.html#fusion.notes.adt_attribute_proxy

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/